MRI-based tumor shrinkage patterns after early neoadjuvant therapy in breast cancer: correlation with molecular subtypes and pathological response after therapy

Background MRI-based tumor shrinkage patterns (TSP) after neoadjuvant therapy (NAT) have been associated with pathological response. However, the understanding of TSP after early NAT remains limited. We aimed to analyze the relationship between TSP after early NAT and pathological response after therapy in different molecular subtypes. Methods We prospectively enrolled participants with invasive ductal breast cancers who received NAT and performed pretreatment DCE-MRI from September 2020 to August 2022. Early-stage MRIs were performed after the first (1st-MRI) and/or second (2nd-MRI) cycle of NAT. Tumor shrinkage patterns were categorized into four groups: concentric shrinkage, diffuse decrease (DD), decrease of intensity only (DIO), and stable disease (SD). Logistic regression analysis was performed to identify independent variables associated with pathologic complete response (pCR), and stratified analysis according to tumor hormone receptor (HR)/human epidermal growth factor receptor 2 (HER2) disease subtype. Results 344 participants (mean age: 50 years, 113/345 [33%] pCR) with 345 tumors (1 bilateral) had evaluable 1st-MRI or 2nd-MRI to comprise the primary analysis cohort, of which 244 participants with 245 tumors had evaluable 1st-MRI (82/245 [33%] pCR) and 206 participants with 207 tumors had evaluable 2nd-MRI (69/207 [33%] pCR) to comprise the 1st- and 2nd-timepoint subgroup analysis cohorts, respectively. In the primary analysis, multivariate analysis showed that early DD pattern (OR = 12.08; 95% CI 3.34–43.75; p < 0.001) predicted pCR independently of the change in tumor size (OR = 1.37; 95% CI 0.94–2.01; p = 0.106) in HR+/HER2− subtype, and the change in tumor size was a strong pCR predictor in HER2+ (OR = 1.61; 95% CI 1.22–2.13; p = 0.001) and triple-negative breast cancer (TNBC, OR = 1.61; 95% CI 1.22–2.11; p = 0.001). Compared with the change in tumor size, the SD pattern achieved a higher negative predictive value in HER2+ and TNBC. The statistical significance of complete 1st-timepoint subgroup analysis was consistent with the primary analysis. Conclusion The diffuse decrease pattern in HR+/HER2− subtype and stable disease in HER2+ and TNBC after early NAT could serve as additional straightforward and comprehensible indicators of treatment response. Trial registration: Trial registration at https://www.chictr.org.cn/. Registration number: ChiCTR2000038578, registered September 24, 2020. Supplementary Information The online version contains supplementary material available at 10.1186/s13058-024-01781-1.


Introduction
Neoadjuvant therapy (NAT) has become the important treatment for locally advanced breast cancers, and patients who achieve pathologic complete response (pCR) after NAT demonstrate improved prognosis and survival [1][2][3].However, due to the high heterogeneity of breast cancer, the efficacy of NAT varies significantly among individuals [4].Early monitoring NAT response of tumors is important for timely adjustment of treatment regimens to optimize efficacy, avoid unnecessary adverse effects and increase disease-free survival [5,6].
Dynamic contrast enhanced (DCE) MRI is a highly precise imaging technique that permits evaluation of a viable tumor before and after NAT by detecting changes in tumor vascularity [7,8].Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria [9] defines tumor response based on the decrease in the longest tumor diameter relative to the pretreatment baseline measurement.However, tumors exhibit various patterns of shrinkage as a result of intricate processes such as necrosis, fibrosis, inflammation, and other internal changes following NAT [10][11][12].In breast cancer, the presence of diffuse nonmass enhancement on the pretreatment MRI or scattered foci within a fibrotic region on the posttreatment MRI poses a challenge to accurately predicting pCR using size measurements [13,14].
Several studies have been conducted to investigate the relationship between tumor shrinkage patterns (TSP) and treatment response [11,15].It has been observed that concentric and fragmented shrinkage patterns are more commonly observed in patients achieving pCR, while stable disease is noted in those who do not achieve pCR during the middle stage and after NAT [16,17].Furthermore, analyses have demonstrated variations in TSP among different subtypes [18][19][20].However, the understanding of TSP following early treatment (i.e., the first or second cycle of NAT) and their association with treatment response remains limited.Given that the alteration in tumor size following early treatment does not consistently provide reliable pCR prediction [21][22][23], we propose the hypothesis that early TSP may serve as an alternative imaging indicator for pCR prediction.This approach offers the advantage of being easily interpretable and applicable in clinical settings.
In this prospective study, we performed longitudinal breast DCE-MRI before and after early NAT to describe TSP and investigate its role as a predictor of therapeutic response.Since the NAT regimens and pCR rate differed among different molecular subtypes, we performed stratified analysis according to molecular subtype.

Participants
In this prospective, single-center, observational study, 362 participants with primary invasive ductal carcinoma who performed pretreatment DCE-MRI were enrolled.Participants eligible for our study included women with invasive breast tumors 1.0 cm or larger at imaging examination who were planning to undergo NAT.Participants with evidence of distant metastasis or progressive diseases during NAT that resulted in changing the initial NAT regimen or surgery cancellation were excluded.Our institutional review board approved this study and each participant provided written informed consent.
This study involved conducting DCE-MRI examinations at three specific timepoints during NAT, including pretreatment (referred to as Pre-MRI), after the first cycle of NAT (referred to as 1st-MRI), and/or after the second cycle of NAT (referred to as 2nd-MRI).The decision to perform Pre-MRI and 2nd-MRI was made by clinicians [24], while 1st-MRI was additionally recommended by clinicians for earlier efficacy evaluation, and its execution was contingent upon the individual preferences of participants.
Participants who performed DCE-MRI before and after early NAT (either 1st-MRI or 2nd-MRI usable) were used as the primary analysis cohort to describe TSP and investigate the value as an early pCR predictor.The primary analysis was an "intention-to-diagnose analysis" based on the total cohort of randomized participants.If a participant performed both 1st-MRI and 2nd-MRI, 2nd-MRI data of the participant were used for primary analysis.
To further analyze TSP after 1st-MRI or 2nd-MRI, we conducted a subgroup analysis to determine the earliest timepoint at which TSP worked.The subgroup analysis was an "per-protocol analysis" based on complete 1st-MRI or 2nd-MRI data (referred to as 1st-timepoint and 2nd-timepoint subgroup analysis, respectively).Participants enrollment flowchart and the cohorts for the primary analysis and subgroup analysis are shown in Fig. 1.

Treatment protocol
All participants received standard six or eight cycles of NAT before surgery according to the National Comprehensive Cancer Network guideline [7].The NAT regimens were based on anthracycline, taxane, or both anthracycline and taxane.For human epidermal growth factor receptor 2 (HER2)-positive tumors, anti-HER2 targeted trastuzumab (H) or trastuzumab + pertuzumab (HP) were added to the chemotherapy drugs.

Imaging analysis
All breast MRI examinations were performed on a 3.0T MR scanner (SIGNA ™ Pioneer, GE Healthcare, Milwaukee, WI, USA) in the prone position using a dedicated 8-channel phased-array breast coil.T1-weighted (T1W) DCE-MRI sequence in the axial plane with temporal resolution of 19.4 s was obtained using three-dimensional (3D) DISCO and fat suppression technique.The scanning parameters were as follows: repetition time/echo time (TR/TE) = 4.9/1.7 ms, flip angle = 10°, field of view (FOV) = 360 × 360 mm, acquisition matrix = 256 × 256, slice thickness/gap = 1.4 mm, number of sections = 116/ phase, acceleration factors = 2.After the pre-contrast scanning followed by a pause of 20 s, the contrast agent was injected intravenously as a bolus (0.1 mmol/kg body weight) by a power injector at 2 mL/s followed by a 20 mL saline flush.Subsequently, 16-20 phase post-contrast images were acquired.Additional imaging protocol details can be found in our previous publication [25].
The assessment of TSP was conducted through a comprehensive analysis of the initial, peak and late post-contrast phases (specifically, the 5th, 7th and 16th post-contrast phases) of DISCO DCE-MRI according to the time intensity curve [11,17].We divided TSP into four groups based on Fukada et al. 's study [11]: concentric shrinkage (CS), diffuse decrease (DD), decrease of intensity only (DIO), and stable disease (SD).The CS pattern was further divided into three types: the simple CS, CS to small foci and CS plus decreased enhancement.The DD pattern was further divided into two types: concentric shrinkage with surrounding lesions, residual multinodular lesions (Figs. 2, 3).All image analyses were independently evaluated by two breast radiologists (W.M.F. and D.S.Y.), with 5 and 10 years of experience, respectively.In cases of inconsistent decisions, resolution was reached through consultation between two radiologists.If the two radiologists were unable to reach a decision after consultation, a third radiologist (Z.L.N., with 20 years of experience) made the final decision.They were blinded to tumor clinicopathological information.
For Pre-MRI, tumor maximum diameter was measured on the axial plane at peak phase.If multiple lesions were present, the largest tumor was selected as the targeted lesion.For follow-up images (1st-MRI or 2nd-MRI), the distance between the two farthest lesions was measured as the maximum diameter of the residual tumors for the DD pattern, while for the other patterns, the maximum diameter was measured consistently with CS with surrounding lesions (pretreatment: a segmental 100 mm non-mass, early NAT: the main lesion showed CS with peripheral focal lesions), d DD: shrinkage with residual multinodular lesions (pretreatment: a diffuse 75 mm non-mass, early NAT: tumor splits into uniform small fragments mixed with fibrous stroma), e decrease of intensity only (DIO) (pretreatment: a regional 60 mm non-mass, early NAT: the degree of enhancement was obviously reduced but unchanged size), f stable disease (SD) (pretreatment: a diffuse non-mass, early NAT: no changes).No CS to small foci non-mass lesions in our study the baseline.For the primary analysis, the tumor size before and after early NAT was recorded as D pre and D early , and tumor size on 2nd-MRI was used as D early for participants who performed both 1st-MRI and 2nd-MRI.The percentage changes (Δ%) in tumor size after early NAT was calculated using the following equation: ΔD early % = (D pre − D early )/D pre × 100%.For subgroup analysis, tumor size measured on 1st-MRI and 2nd-MRI was recorded as D 1st and D 2nd , respectively.The Δ% on 1st-MRI and 2nd-MRI was calculated using the following equation: The mean value of tumor size measured by both readers was used for the final analysis.Additionally, tumor morphological and kinetic features were analyzed according to the 5th Ed.Breast Imaging Reporting and Data System (BI-RADS) lexicon [26].

Histopathology
All patients received a core-needle biopsy guided by ultrasonography before NAT.The pathological specimens were viewed and diagnosed by a breast pathologist with more than 20 years of experience in breast pathologic examination.Immunohistochemistry (IHC) was performed for each patient to determine the baseline estrogen receptor (ER), progesterone receptor (PR), HER2 status, and Ki-67 index.According to ASCO guideline [27], the cutoff value for ER and PR was set at 1%, and the cutoff value for Ki67 was 20%.Regarding HER2 status, tumors with an IHC staining of 0 to 1+ were defined as HER2 negative and 3+ as HER2 positive.Fluorescence in situ hybridization was conducted when HER2 expression was detected as 2+ on IHC.A non-amplified FISH result denotes the HER2 status as negative, and an amplified result denotes the HER2 status as positive.Based on ER, PR, and HER2 status, the biological subtypes included the following: hormone receptor (HR) + /HER2 − (ER + and/or PR + and HER2 − ), HER2 + (HER2 + regardless of HR status) and triple-negative breast cancer (TNBC: ER − , PR − , and HER2 − ).

Definition of histologic therapeutic effects
Postoperative pathological response was graded based on the Miller-Payne grading system [28].pCR was defined as ypT0 or ypTis with no residual invasive tumor (Miller-Payne grade 5, residual ductal carcinoma in situ could be present).Patients with Miller-Payne grades 1 or 2 were classified into the nonresponse group (pNR), and patients with grades 3, 4, or 5 were in the response group (non-pNR) (Table 1).The histopathologic status of the axillary lymph nodes was not considered in pCR definition.

Statistical analysis
Mann-Whitney and Chi-square (or Fisher's exact) tests were used to compare the differences in clinicopathological and imaging features between the pCR and non-pCR groups (or pNR and non-pNR groups in HR + / HER2 − subtype).To compare TSP in different treatment response groups, the Chi-square test and Bonferroni correction for multiple comparisons were used, with a p value < 0.00833 (p < 0.05/6) considered statistically significant.The inter-reader agreement between both readers for TSP was calculated using Cohen's Kappa (κ).
Clinicopathologic and imaging features potentially predictive for pCR were analyzed using binary logistic regression.Factors with a p value of < 0.10 on univariate logistic regression were entered into multivariate logistic regression and a p value < 0.05 was statistically significant.Performance for predicting pCR was assessed with the area under the receiver operating characteristic curve (AUC), accuracy, sensitivity, specificity, positive and negative predictive values (PPV and NPV).All analyses were performed using Statistical Package for the Social Sciences (SPSS, version 25.0, IBM Corporation, Armonk, NY, USA) and MedCalc (version 15.6.1).

Participants characteristics
A total of 362 consecutive participants from September 2020 to August 2022 were enrolled in our study.Eighteen (5.0%) of 362 participants were excluded due to  1).Baseline characteristics for the primary analysis cohort and two subgroup analysis cohorts are listed in Table 2.The most common molecular subtype was HR + /HER2 − (151/345, 44%) followed by HER2 + (123/345, 36%) and TNBC (71/ 345, 21%).After NAT, 113/345 (33%) achieved pCR.No significant difference was found in the primary analysis cohort versus the two subgroup analysis cohorts across all characteristics (Table 2, all p > 0.05).
In the primary analysis cohort, pCR tended to present with high histologic grade, low D early and large change in tumor size (p < 0.001).Molecular subtype and NAT regimen showed a significant association with pCR (p < 0.001).No significant difference was detected between participants with pCR and non-pCR in terms of age, baseline tumor size, menopausal status, clinical TNM stage and other MRI characteristics (Additional file 1: Table S1, all p > 0.05).The participants characteristics in subgroup analysis cohorts were consistent with those of the primary analysis cohort (Additional file 1: Table S2).
In 1st-timepoint subgroup analysis, multivariate analysis showed that the DD pattern (OR = 9.99; 95% CI 1.78-56.04;p = 0.009) predicted pCR independently of the change in tumor size (OR = 0.87; 95% CI 0.46-1.64;p = 0.659) in HR + /HER2 − subtype.In HER2 + subtype, univariate analysis showed that the SD pattern and change in tumor size were associated with pCR; multivariate analysis showed that the change in tumor size was the only independent factor to predict pCR (OR = 1.75; 95% CI 1.20-2.56;p = 0.004).In TNBC, univariate analysis showed that the SD pattern (OR = 0.05; 95% CI 0.01-0.45,p = 0.007) and change in tumor size (OR = 1.94; 95% CI 1.29-2.92;p = 0.001) were associated with pCR, but the differences were not statistically significant in multivariate analysis (Additional file 1: Table S9).The result of complete 1st-timepoint analysis was consistent with the primary analysis.

Early imaging response strategy map
Strategy maps based on TSP and the change in tumor size in each subtype are plotted.For pCR prediction in HR + /HER2 − subtype, radiologists should first identify the non-pCR patients with the SD pattern (or a few DIO or CS plus decreased enhancement patterns) and the simple CS pattern; Then evaluate whether the patient has the DD pattern, which is a potential pCR manifestation although there is only 38% likelihood of pCR.In HER2 + and TNBC, we should first identify a pCR patient with the CS to small foci pattern or a non-pCR patient with the SD pattern; If neither, the likelihood of pCR depends on the tumor size change with OR of 1.86 in HER2 + and 1.94 in TNBC for 10% increment at 1st-timepoint, for example (Fig. 5).
For pNR prediction in HR + /HER2 − subtype, radiologists should first identify whether the patient has the DD pattern, which is a highly likely non-pNR manifestation.If the patient does not have the DD pattern, the likelihood of pNR depends on the tumor size change with OR of 0.63 for 10% increment at 1st-timepoint, for example (Fig. 6).

Table 4
Univariate and multivariate analysis of factors associated with pCR according to different molecular subtypes in the primary analysis cohort pCR pathologic complete response, NAT neoadjuvant therapy, HER2 human epidermal growth factor receptor 2, TNBC triple-negative breast cancer, HR hormone receptor, CS concentric shrinkage, DD diffuse decrease, DIO decrease of intensity only, SD stable disease, OR odds ratio, Ref reference, NA not applicable, D pre the tumor size at Pre-MRI, D early the tumor size after early NAT, ∆D early % the percentage changes in tumor size after early NAT (continuous variable for 10% increment) a D pre and D early were analyzed only in univariate analysis b ∆Dearly% with higher OR was used in multivariate analysis; NA was due to the fact that the shrinkage pattern had zero samples in either the pCR or non-pCR group

Discussion
In the modern era with updated neoadjuvant therapy regimens, the present study evaluated TSP on DCE-MRI after early NAT and its association with pCR within each breast cancer subtype.Our findings indicated that the DD pattern after early NAT, particularly at 1st-timepoint, was a tumor response marker independent of the size change in HR + /HER2 − subtype; the SD pattern in HER2 + and TNBC after early NAT strongly indicated non-pCR.TSP could serve as additional straightforward The classification and definition of TSP at MRI have not been consistently recognized and unified.The CS or non-CS patterns after NAT and further refinement of non-CS pattern at mid-NAT were commonly used [11,17,20].Based on Fukada et al. 's study [11], we developed four-category TSP and subdivided CS and DD pattern to suit early NAT response.The overall loss of cellularity after NAT was not always reflected by a decreased tumor size.NAT can cause different changes in the nucleus and cytoplasm of tumors, leading to changes in overall morphology and exhibiting different TSP [29].Compared to HER2 + subtype, HR + /HER2 − subtype tends to grow slowly, showing low apoptosis rates and genetic instability [11].The internal heterogeneity of these tumors causes them to shrink inconsistently and crumble into small foci or scattered cells.The sparse microvascular distribution in HR + /HER2 − subtype also leads to uneven drug delivery, which tends to have the DD pattern after NAT.In our study, the DD pattern after early NAT tended toward pCR in HR + /HER2 − subtype, mainly at 1st-timepoint,  [20] reported that early fragmentation pattern after 2 months neoadjuvant endocrine therapy suggested effective treatment in ER + / HER2 − subtype.The DD pattern may be the early manifestation of HR + /HER2 − subtype response to NAT earlier than size reduction.Our study recommended introducing the TSP for early imaging response strategies in HR + / HER2 − subtype.
HER2 + and TNBC have the highest proportion of CS pattern, which is consistent with previous studies for mid-and post-NAT evaluation [30][31][32][33].Animal studies [34] on tumor subregions have shown that tumor margins of HER2 + and TNBC are distributed with abundant microvessels and high cell proliferation.Abundant vessels facilitated the delivery of drugs thus making these tumors more sensitive to therapy, resulting in more homogeneous cell reduction and shrinkage.Heacock et al. [18] and Eom et al. [19] reported that the CS pattern was a stronger predictor of pCR in HER2 + and TNBC after NAT.Different from post-NAT timepoint, the CS pattern after early NAT did not show a significant pCR tendency compared with the DD pattern, but the SD pattern strongly indicated non-pCR in HER2 + and TNBC.The change in tumor size was still a strong predictor of pCR in HER2 + and TNBC after early NAT, even at 1st-timepoint.
Based on the observed TSP, we develop an early imaging response strategy for each subtype of breast cancer.By employing this strategy, clinicians can effectively inform patients of the potential pathological response and its associated probability.The results of 1st-timepoint subgroup analysis were consistent with those of the primary analysis cohort, indicating TSP can be evaluated even after the first cycle of NAT.This easily understandable approach can assist clinicians in modifying treatment plans to enhance effectiveness, minimize unnecessary adverse effects, and improve disease-free survival rates.However, noted that the signal intensities of DCE-MRI are influenced by imaging protocols and gadoliniumbased contrast agents from different vendors, therefore TSP such as "DIO" and "CS plus decreased enhancement" may be susceptible to potential influences.To mitigate the variability in TSP evaluation after treatment, it is crucial to utilize uniformity MRI scanners, standardized contrast agents, and skilled radiologists in the serial imaging evaluation of the identical patient during NAT.
Our study had some limitations.First, despite the overall large sample size, the number of each subtype was limited.Enhancing the sample size for each subtype would augment the strength of our evidence.Secondly, the homogeneity of the study sample and the data acquisition method mitigated the influence of confounding variables, but the result may be specific to this acquisition technique.The performance of our findings on a different scanner platform, or with different imaging protocol is unknown.Finally, our study employed visual assessment conducted by radiologists, which was both qualitative and subjective.Future research should strive to incorporate artificial intelligence techniques to enable rapid, objective and reproducible analysis of TSP.

Conclusion
The TSP after early NAT may serve as an additional straightforward and comprehensible indicator of treatment response in addition to the change in tumor size.Specifically, the diffuse decrease pattern in HR + /HER2 − subtype is a tumor response marker independent of the size change, and the stable disease in HER2 + and TNBC strongly indicates non-pCR at 1st-timepoint.

Fig. 4 A
Fig. 4 A Invasive ductal carcinoma (HR + /HER2 − ) with pathologic complete response (pCR) after NAT in a 49-year-old woman: (a) pretreatment: a 71 mm mass occupying most glands in the upper right quadrant; (b) early neoadjuvant therapy (NAT): the lesions showed shrinkage with residual multinodular lesions (DD pattern).Invasive ductal carcinoma (HR + /HER2 − ) with non-pCR after NAT in a 70-year-old woman: (c) pretreatment: a 25 mm mass in the upper right quadrant; (d) early NAT: the lesions showed the simple concentric shrinkage (CS pattern) with a diameter reduction of 4 mm.B Invasive ductal carcinoma (HER2 + ) with pCR after NAT in a 44-year-old woman: (a) pretreatment: a 45 mm mass in the upper left quadrant; (b) early NAT: the lesion size was notably diminished with only residual enhancement foci (CS: CS to small foci pattern).Invasive ductal carcinoma (HER2 + ) with non-pCR after NAT in a 58-year-old woman: (c) pretreatment: a 29 mm mass in the upper left quadrant; (d) early NAT: the lesions showed the stable disease (SD pattern) with no changes in size or morphology.C Invasive ductal carcinoma (TNBC) with pCR after NAT in a 53-year-old woman: (a) pretreatment: a 43 mm mass in the upper left quadrant; (b) early NAT: the lesions showed the simple concentric shrinkage (CS pattern) with a diameter reduction of 13 mm.Invasive ductal carcinoma (TNBC) with non-pCR after NAT in a 65-year-old woman: (c) pretreatment: a 50 mm mass in the upper right quadrant; (d) early NAT: the lesions showed the stable disease (SD pattern) with no changes in size or morphology

Fig. 5 Fig. 6
Fig. 5 Strategy map for predicting pCR based on shrinkage patterns and the change in tumor size in each subtype

Table 1
Miller-Payne grading system A marked disappearance of tumor cells such that only small clusters or widely dispersed individual cells remain; more than 90% loss of tumor cellsGrade 5No malignant cells identifiable in sections from the site of the tumor; only vascular fibroelastotic stroma remains often containing macrophages.However, ductal carcinoma in situ (DCIS)

Table 2
Participants characteristics

Table 2
(continued)Unless otherwise specified, data are numbers of participants, with percentages in parentheses p values show the results of comparisons between the participants in the subgroup cohorts versus the participants in the primary analysis cohort pCR pathologic complete response, HER2 human epidermal growth factor receptor 2, TNBC triple-negative breast cancer, HR hormone receptor, NAT neoadjuvant therapy, BPE background parenchymal enhancement, FGT fibroglandular tissue a 345 MRI including 138 1st-MRI and 207 2nd-MRI

Table 3
MRI-based tumor shrinkage patterns association with pCR according to different molecular subtypes in the primary analysis cohort pCR pathologic complete response, HER2 human epidermal growth factor receptor 2, TNBC triple-negative breast cancer, HR hormone receptor, CS concentric shrinkage, DD diffuse decrease, DIO decrease of intensity only, SD stable disease, NA not applicable a Following adjustment for multiple comparisons with Bonferroni's correction, the statistically significant p values were annotated (p < 0.00833)